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Onondaga County authorities inspect the bus that crashed into a railway bridge. ((Peter Chen/Post-Standard/Associated Press))

The driver of a double-decker Philadelphia-to-Toronto bus that crashed into a railway bridge in upstate New York took a wrong turn and was unfamiliar with the road, the Onondaga County sheriff says.

The Megabus vehicle slammed into the bridge over the Onondaga Lake Parkway in Salina, a suburb of Syracuse, N.Y., around 2:30 a.m. ET Saturday, killing four people and injuring several others — including seven Canadians from Ontario.

Sheriff Kevin Walsh says the driver, who remains in hospital with head injuries, missed a turn en route to a scheduled stop in Syracuse and was not sure where he was.

The driver was supposed to exit Interstate 81 and stop for half an hour at the Regional Transportation Center on Park Street in Syracuse before making another stop in Buffalo, according to an official at Coach USA, which operates Megabus. Instead, the driver ended up on the lakeside parkway.

Walsh says the bus was too tall and wasn't able to clear the low-hanging span. There were flashing signs warning that the bridge clearance was 10 feet 9 inches (3.27 metres).

The bus was about 13 feet in height (3.96 metres).

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The bus flipped onto its side under the the railway bridge in a suburb of Syracuse, N.Y. ((Peter Chen/Post-Standard/Associated Press))

Company officials say the driver, John Tomaszewski, 59, of New Jersey, has been working at Megabus since last June. They say he was familiar with Syracuse.

Branislav Nikolic, 25, and his wife Masa Nikolic, 24, of Toronto were on the top level of the bus, in the middle, making their way home after visiting relatives in Philadelphia.

"Basically before the accident I was asleep and my wife was across from me on the next set of seats," the York University student said. "We were both sleeping, and I heard this horrible crashing. We had hit the overpass."

Two Americans died in the accident, along with a passenger from Malaysia and one from India.

Twenty passengers, including the seven Canadians, were treated and released from hospital, said Don Carmichael, a senior vice-president at Coach USA.

One passenger was in very critical condition and four others, including the driver, were in serious condition, but were expected to survive, the sheriff said.